The drilling and construction of wells, for example for the hydrocarbon exploration and production industry, includes many different operations which involve the pumping of fluids from surface through the wellbore and back to surface. A drilling operation typically involves the rotation of a drill bit on the end of a drill string (or drill pipe), which extends from a drilling platform to a drill bit. Drilling fluid (referred to as drilling mud) is pumped from one or more pits on a drilling rig down through the drill string to the drill bit to fulfil a number of different functions, including providing hydrostatic pressure to control the entry of fluids from the formation into the wellbore, lubricating the drill bit, keeping the drill bit cool during drilling, and carrying particulate materials such as drill cuttings upwards and out of the well away from the drill bit. Drilling fluid and cuttings emanating from the wellbore are carried up the annular space between the wall of the bore being drilled and the drill pipe to the mudline. In conventional subsea drilling, a riser is installed above a blow-out preventer (BOP) stack on top of the wellhead, and extends to the surface. Drilling fluid and cuttings are returned to the drilling rig for processing, re-use, storage, removal and/or treatment through the annulus between the drill pipe and the riser.
Typically the drilling fluid system is a closed-loop system, which has a known well volume through which the drilling fluid is circulated, and one or more drilling fluid or “mud” pits on the drilling rig. The rig crew monitors the level of drilling fluid in the pit to detect unwanted influx of reservoir fluids (including gases) into the wellbore, referred to as a “kick”. The rig crew responds to kicks by adding one or more barriers to control the influx and circulate the additional fluid out of the wellbore and prevent uncontrolled flow of fluids into the well. Parameters monitored include “pit gain”, which is the difference between the volume of fluid pumped into the well and the volume of fluid pumped out of the well. In a closed-loop system for a stable well, the two values should be equal, whereas a positive pit gain will indicate an influx of reservoir fluid and a pit loss will indicate a loss of drilling fluid into the formation.
For a single pit drilling system, pit gain can be determined by monitoring the level of drilling fluid in the pit. Active pit systems are computer-controlled systems which enable several pits to be aggregated into one “active pit volume”, which can be treated as a single pit for monitoring pit gain.
When a production well reaches the end of its economic or technical viability, it may be necessary to temporarily or permanently plug and abandon (P&A) the well to establish a permanent barrier against the flow or migration of hydrocarbons to the surface. Plug and abandonment methodologies are varied, but conventionally use a drilling rig (such as a jack-up rig installation) to install a blowout preventer (BOP) stack and marine riser on the well. The production tubing is cut and pulled to surface to enable one or more cross-sectional barriers or plugs to be installed in the wellbore. During the Pulling Out of Hole (POOH) of the tubing, the drilling fluid circulation system of the rig is used to provide drilling fluid from the pit, via the marine riser, to the wellbore to compensate for the loss of volume as the tubing is removed from the well. During POOH of the production tubing, pit gain can be monitored at surface to determine whether there is an influx or outflow of fluid which is indicative of a problem with the seal or seals provided by the plugs. The BOP stack provides full control of wellbore fluids and enables any unwanted flow of reservoir fluids into the annulus to be mitigated against.
Methods which rely on the use of drilling rigs are expensive and time-consuming to mobilise. These disadvantages, coupled with problems associated with the lack of availability of drilling rigs, have led to a number of new proposals for rig-less plug and abandonment operations which utilise vessels rather than drilling rigs. Vessels do not commonly have a marine riser, and so to utilise vessels for plugging and abandonment requires new ‘riser-less’ techniques to be developed.
It has also been proposed to use coiled tubing systems in plugging and abandonment operations, to mitigate the reliance on drilling rig deployment and to enable the operations to be controlled from a vessel such as a lightweight intervention vessel (LWIV). However, a coiled tubing intervention, in the absence of deployment through a marine riser, does not provide a return annulus for drilling fluids and does not enable volume control as the tubing is removed from the well.
It is known to provide drilling fluid collection, handling and return equipment in subsea drilling operations which do not use conventional marine risers. For example, when drilling the uppermost section of the wellbore, which is referred to as the “tophole” is drilled, there is no riser pipe installed between the seabed and the drilling rig, and as there is no return path for drilling fluids from the wellbore back to the surface, the drilling mud and cuttings are conveyed to surface via a dedicated return line. One such system is described in U.S. Pat. No. 4,149,603 [1], and uses a riserless mud return system including a hose, separate from the drill string, to carry mud to the surface. A pumping means is used to pump mud through the hose back to surface, with the pump operated in dependence on the detected level of mud and cuttings supported within a mud sump. Additional examples of systems which pump drilling fluids to surface via dedicated return lines are disclosed in US 2008/0190663 and the applicant's co-pending international publication numbers WO 2012/140446 and WO 2012/156742.
There is generally a need for a method and apparatus which addresses one or more of the problems associated with conventional plugging and abandonment techniques when used from vessels.
It is amongst the aims and objects of aspects of the invention to provide a method and/or apparatus for controlling the volume of a fluid in a subsea wellbore system which obviates or mitigates one or more drawbacks or disadvantages of the prior art. It is an aim of at least one aspect of the invention to provide a method and apparatus for the plugging and abandonment of subsea hydrocarbon wellbores. A further aim of at least one aspect of the invention is to provide a vessel-based method and apparatus for controlling the volume of fluid during a plugging and abandonment operation on a subsea hydrocarbon well, which may be performed from a LWIV and without relying on a drilling rig and/or marine riser system.
It is another aim and object of an aspect of the invention to provide a method and apparatus for controlling the re-filling of a subsea hydrocarbon well from a dedicated well fluid hose during a plugging and abandonment operation.
Further aims and objects of the invention will become apparent from reading the following description.